The present invention relates generally to DC step-up voltage converters particularly of the type wherein the output is rectified to provide DC to DC conversion typically used to supply a high DC operating voltage for an electronic photographic flash tube with power supplied from low voltage DC batteries. Numerous circuit configurations for supplying the DC to DC conversion function have been provided in the past with emphasis given to continually improving the light weight and low power consumption characteristics of such devices in order that small portable and self-contained units may be manufactured and sold at reasonable prices.
In the prior art circuits the conventional arrangement has been to provide a blocking type oscillator in the form of a transformer having three windings one of which is the load for an oscillating active element such as a transistor with a secondary winding of a large number of turns providing a substantial step-up in voltage in accordance with the primary-secondary turns ratio. The third winding on the transformer is employed to provide the feedback in association with time constant circuits to permit the transistor-transformer combination to operate as a blocking oscillator. Such arrangements generally have a variable frequency during the charging cycle as the current loading on the oscillator varies as the output voltage approaches the maximum level. Thus the transformer cannot be optimized as to efficient energy transfer at constant frequency and duty cycle and the corresponding size and cost are not optimally minimum. Typical of these and similar prior art circuits are found in U.S. Pat. Nos. 3,310,723, 3,831,079 and 3,917,975. While two winding transformers (i.e., a primary and secondary winding without a third feedback winding) have been employed in photo-flash type circuits, e.g., U.S. Pat. No. 3,679,991, such arrangements have sometimes involved a connection to the high voltage secondary winding and an oscillating transistor connected to the primary winding with interconnection of the two to provide the necessary feedback for oscillation. Because of the high voltages involved in such photo-flash converters, it is generally desirable to isolate the secondary high voltage circuit.
In other such circuits the functions of the oscillator and power drive for the transformer cannot be independently optimized. Typically, a duty cycle feedback control is employed which varies the "mark-space" duty cycle such that the transformer is not matched to a constant steady-state duty cycle for the duration of the charging cycle (cf Electronics pp 95-7 Mar. 6, 1975 and Motorola Semiconductor Products, Inc. Application Note AN-442). In order to conserve battery drain in such circuits, germanium transistors are ordinarily used and a hysteresis effect employing a neon tube is generally relied upon to switch the circuit between active oscillation for charging the capacitor and a reduced conversion level state during the time the output voltage is high enough to maintain discharge in a neon tube connected across a portion of the output voltage. The present invention utilizes silicon transistors adapted to monolithic construction and low loss in a stand-by off condition of very low current thus achieving economy of cost and conservation of battery energy with the on-off condition controlled by an accurate hysteresis voltage comparator. While voltage comparator circuits more accurate than neon tubes have been employed in the prior art, e.g., U.S. Pat. No. 3,863,128, arrangements such as these generally employed two reference zener diode devices or the like for establishing the two levels for control of the oscillator and thus unduly add to the complexity and expense of such arrangements.